This invention relates in general to electrical systems and in particular to reliable multiple path flexible circuit connections.
In many industrial systems, such as computer and telecommunications systems, there is a need for making a large number of electrical interconnections between a plurality of points of interest. It is generally desirable to achieve reliable connections (i.e. with low connection failure rates), with good electrical properties, and to be able to dispose such connections within a small area to address space limitations. Various prior art approaches have been attempted which experience limitations in one or more of the above-mentioned characteristics.
One prior art approach is that of Printed Wire Board (PWB) flex circuits. PWB flex circuits are generally easy to manufacture if they have wide lines and traces. However, such circuits are generally difficult to manufacture in volume when fine lines, high density, and/or tight impedance controls are required. Moreover, if tight impedance controls are required, yield losses are generally high in volume production due primarily to variations in the etch processes.
FIG. 1 is a section view of a strip line flex circuit 100 for high speed high density applications according to a prior art solution. Conductive traces 102 are shown with a dashed line. Ground planes 101 and 103 are shown running parallel to traces 102.
FIG. 2 is a top view of a center line cross section of the strip line flex circuit depicted in FIG. 1. A number 200 of parallel traces 201 may be seen in the top view of FIG. 2. Generally, the width of and spacing between traces 201 may be important features in determining electrical properties of the structure such as characteristic impedance, resistance, skin effect losses, and crosstalk between the traces.
FIG. 3 depicts sectional views 300 of four possible conductive trace geometries. The three columns separated by ideal spaces 306 are generally equivalent. Accordingly, the four trace geometries depicted in the rightmost column 301 will be discussed herein. Trace 302 generally represents an ideal trace geometry, which although highly desirable, is very difficult to achieve in a typical etch process. Trace 303 generally represents a desirable trace geometry which may be achieved with careful process control and with some yield loss. Trace 304 depicts a trace from which an excessive amount of conductive material, such as copper, has been removed. Trace 304 will generally experience excessive direct current (d.c.) resistance, increased skin effect losses, and undesirably high characteristic impedance. Element 305 depicts an under-etched trace. Such an under-etched trace will generally have excessive crosstalk and lower than ideal characteristic impedance.
High speed data cables have been employed to provide interconnection having superior electrical signal characteristics. However, the use of such cabling is generally more expensive to implement for transmission of a given set of signals than is printed wire board flex circuit. Moreover, connectors used to connect such data cables to a board generally provide lower signal density than do flex circuits. Furthermore, such cable connectors are commonly the cause of impedance mismatches, crosstalk, and skew.
In certain cases, multiple rigid PCBs (printed circuit boards) are used to establish electrical connections to system or subsystem units which are not on the same plane. As a result, multiple connectors may be introduced into the signal path, the addition of which generally degrades the quality of signal transmission. In addition, implementing a plurality of rigid PCBs generally adds to system cost, and takes up additional space.
FIG. 4A is a section view 400 of a via connected to a wire according to a prior art multiwire connection arrangement. Employing this arrangement, via 401 is generally formed onto the end of wire 402. Wire 402 may be coated with TEFLON(copyright) (polytetrafluoroethylene) for insulation purposes. In a connection employing the arrangement depicted in FIG. 4A, the available area for establishing a connection between wire 402 and via 401 is generally limited to the cross-sectional area of wire 402. Some additional contact area may become available where the polytetrafluoroethylene coating is etched back for a finite distance along wire 402 from the outside diameter of via 401. FIG. 4B is a top view of the same connection.
Although the embodiment depicted in FIGS. 4A and 4B generally provides the superior electrical properties of discrete wiring, the attachment of wire 402 to via 401 provides a weak mechanical connection between wire 402 and via 401 which is subject to failure if wire 402 is pulled or moved in any direction.
FIG. 5A is a section view 500 of a via 501 connected to a trace 502 in a printed circuit board arrangement. Employing this arrangement, a hole is drilled in a conductive pad connected to trace 502, and plating material added to create via 501. FIG. 5B is a top view of the connection depicted in FIG. 5A. This connection generally provides for a 360 degree connection between plating on via 501 and the copper of trace 502. This trace-via connection offers a more robust mechanical connection than the connection between the discrete wire and via depicted in FIG. 4. However, the trace-via connection of FIG. 5 is subject to the inconsistency in dimensional tolerance and electrical properties discussed in connection with FIG. 3.
FIG. 6 is a section view of a rigid circuit employing discrete wiring 602 between stripline shield layers 601 according to a prior art embodiment. FIG. 7 is a top view 700 of a center line cross section of wires 602 depicted in FIG. 6. Returning to FIG. 6, generally, the cross sectional area between stripline shield layers 601 is filled with rigid dielectric material 603, such as FR4 or G-Tec. The rigid circuit embodiment of FIG. 6 benefits from the advantageous electrical performance properties of discrete wiring. However, the embodiment employs the mechanically vulnerable wire to via connection discussed in connection with FIG. 4A.
Accordingly, it is a problem in the art that PWB flex circuits are difficult to manufacture in high volume and experience and high yield losses due to etch variations.
It is a further problem in the art that it is difficult to generate consistent trace geometries, resulting in inconsistent electrical properties for conductive traces.
It is a still further problem in the art that cables are generally more expensive to implement than flex circuits for the same number of signals.
It is a still further problem in the art that connectors used to attach cables to boards offer lower signal density than PWB flex circuits, and commonly cause impedance mismatches, crosstalk, and skew.
It is a still further problem in the art that the deployment of multiple PCBs cause added cost, take up additional space, and cause degraded performance because of the implementation of multiple connectors along individual signal paths.
It is a still further problem in the art that the connection of discrete wires to plated vias generally mechanically weak and subject to failure when the wire is pulled or moved.
The present invention is directed to a system and method which provides consistently high quality electrical performance characteristics in combination with robust mechanical attachment between conductors and electrical junctions. Preferably, discrete wires are connected to conductive pads employing either laser welded or ultrasonically bonded wire joints between the wires and pads. A plurality of layers having conductive pads may be combined to form a multiple layer flex circuit. Holes may then be drilled through the conductive pads in the traditional manner and then plated to create vias.
Preferably, electrically reliable discrete wires may be employed and still benefit from the mechanically robust connection created by the attachment of the pad to the via. Therefore, there is a generally a two stage connection which includes a connection between a wire, or other high performance conductor, to a conductive pad, and then a robust connection between the conductive pad and the via. A robust connection between the high quality conductor and the conductive pad may be formed by a number of mechanisms including but not limited to: laser welding and ultrasonic bonding. In this manner, a mechanically flexible circuit is provided which enjoys the high quality electrical performance of discrete wiring conduction and robust mechanical attachments between the wire, pad, and via, thereby enabling the circuit to be flexed at will without causing electrical connection failure.
In a preferred embodiment, a wire may be placed in contact with a conductive pad and then be ultrasonically bonded or welded thereto. Thereafter, a hole may be drilled in the conductive pad, and optionally, through a plurality of other layers in a flex circuit. The drilled hole may then be plated, thereby enabling the plating material to form a solid conductive and mechanical connection to the conductive pad. In this manner, the resulting connections are mechanically robust and benefit from the superior electrical properties of discrete wiring.
In a preferred embodiment, the system and method described above allow the creation of a high speed flexible circuit that combines high speed signal handling ability with robust mechanical connections. Since the bulk of the distance traveled by the signals is covered by a high quality conductor, such as, for instance, drawn wire, rather than etched trace, the conductor surface is preferably smooth, thereby enabling low loss skin effect parameters to be realized.
In a preferred embodiment, the wires may be individually formed, thereby enabling such wires to avoid the electrical performance problems associated with under-etch and over-etch to be avoided. Preferably, the wire to pad connection, which may be accomplished by welding or ultrasonic bonding, allows robust signal vias to be created employing standard drill and plating processes. Preferably, tighter impedance, resistance, and skin effect loss parameters may be realized employing a process which allows consistent manufacturing of a large number of circuits.
Therefore, it is an advantage of a preferred embodiment of the present invention that electrical performance characteristics of high quality conductors such as discrete wiring may be combined with robust mechanical characteristics in a flex circuit.
It is a further advantage of a preferred embodiment of the present invention that a flex circuit connection with robust mechanical and electrical characteristics may be reliably and consistently manufactured.
It is a still further advantage of a preferred embodiment of the present invention that impedance mismatch and other electrical performance problems associated with introduction of connectors into the signal path may be minimized.
It is a still further advantage of a preferred embodiment of the present invention that high quality conductors may be employed which do not suffer the yield losses associated with fine line etching of conductive traces.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.